When the viewport is 400px or more, container class should have a grid-template-areas property in which the footer and header areas occupy the top and bottom rows respectively and advert and content occupy the left and right columns of the middle row.

I’ve edited your post for readability. When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make easier to read.

Note: Backticks are not single quotes.

See this post to find the backtick on your keyboard. The “preformatted text” tool in the editor (</>) will also add backticks around text.